An optical signal-to-noise ratio (Optical Signal-to-Noise Ratio, “OSNR” for short) is one of key indicators for measuring optical signal performance in an optical wavelength division multiplexing (Wavelength Division Multiplexing, “WDM” for short) system. With wide deployment of coherent 40/100 Gbit/s wavelength division multiplexing systems, when a channel spacing is 50 GHz or smaller, optical spectrums of adjacent channels overlap, and in a WDM system including a reconfigurable optical add-drop multiplexer (Reconfigurable Optical Add-Drop Multiplexer, “ROADM” for short) during signal transmission, after amplified spontaneous emission (Amplified Spontaneous Emission, “ASE” for short) noise introduced by an erbium-doped optical fiber amplifier (Erbium-doped Optical Fiber Amplifier “EDFA” for short) is filtered by the ROADM, ASE noise levels inside and outside a channel become different. These factors cause a measured value acquired by using a traditional OSNR test method, that is, an out-of-band test method, to be no longer accurate, and an in-band method is required to detect the OSNR.
At present, one of in-band OSNR detection technologies is a polarization extinction (Polarization Nulling or Polarization Extinction) method. The method or its modified method separates an optical signal from noise by optical and algorithm means according to a basic characteristic that the optical signal to be detected is polarized while the noise is unpolarized. Because a coherent system uses a polarization state modulation mode, the optical signal and the noise cannot be distinguished according to a difference between polarization characteristics of the optical signal and the noise. Especially for a 100 G dual-polarized (or called polarization division multiplexing, Polarization Division Multiplexing) signal, the noise and the optical signal almost overlap and cannot be separated, and a measurement error is caused.
Another in-band OSNR detection method that can be applied to the coherent system is to split an input optical signal to be detected into two signals and send the two signals to a photodiode (PD) 1 and a PD2 respectively. One signal to be detected is processed by a low pass filter (Low Pass Filter, “LPF” for short) after being received by the PD1 and the other signal to be detected is processed by a band pass filter (Band Pass Filter, “BPF” for short) after being received by the PD2. These two signals, after being filtered, are sent to an analog to digital converter (Analog to Digital Converter, “ADC” for short) respectively for sampling. Sampled data is sent to a signal processing unit for processing and calculation, and the OSNR of the optical signal to be detected is acquired. A basic principle of the method is: Total signal energy of a phase shift keying (Phase Shift Keying, “PSK” for short) signal in two polarization states is highly concentrated in the vicinity of frequency 0 of a radio frequency (Radio Frequency, “RF” for short) spectrum after the phase shift keying signal is received by a photodiode, and beat frequency components of the signal and the noise may be extracted at a low frequency, and further, a ratio of a total signal size to the ASE noise, that is, the OSNR of the optical signal to be detected, may be calculated.
However, in the foregoing in-band OSNR detection methods, a residual amplitude modulation signal may be superimposed on a corresponding original phase modulation signal without an amplitude fluctuation at a transmitting end. Such amplitude modulation is reflected as the amplitude fluctuation on an RF power spectrum, and is generally called a residual amplitude modulation component (Residual Amplitude Modulation Component). In addition, this residual amplitude modulation component is different for optical signals to be detected that have different modulation formats and/or bit rates, and therefore OSNR detection precision is seriously affected.